On Wed, Jul 05, 2023 at 06:32:30PM +0200, Richard Henderson wrote:
> https://gitlab.com/qemu-project/qemu/-/commit/
> 3307e08c6f142bb3d2406cfbc0ee19359748b51a
FWIW reverting this commit (alone) causes qemu to throw this assertion:
qemu-system-x86_64: ../accel/tcg/tb-maint.c:1096: tb_invalidate_phys_page_range__locked: Assertion `((start ^ last) & ((target_long)-1 << 12)) == 0' failed.
I've seen this bug happen on both very old and very new versions of
qemu (even back to 6.0), so I don't think the bug is caused by or
fixed by any recent change.
Rich.
> r~
>
> On Wed, 5 July 2023, 18:28 Richard W.M. Jones, <[email protected]> wrote:
>
> On Tue, Jul 04, 2023 at 08:46:20AM +0100, Richard W.M. Jones wrote:
> > We have been having the same sort of problem
> > (https://bugzilla.redhat.com/show_bug.cgi?id=2216496).? It's another
> > of those bugs that requires hundreds or thousands of boot iterations
> > before you can see it.? There is a test in comment 27 but it requires
> > guestfish and some hacking to work.? I'll try to come up with a
> > cleaner test later.
> >
> > We see stack traces like:
> >
> > [? ? 3.081939] clocksource: acpi_pm: mask: 0xffffff max_cycles: 0xffffff,
> max_idle_ns: 2085701024 ns
> > [? ? 3.082266] clocksource: Switched to clocksource acpi_pm
> > [? ? 3.090201] NET: Registered PF_INET protocol family
> > [? ? 3.093098] int3: 0000 [#1] PREEMPT SMP NOPTI
> > [? ? 3.093098] CPU: 3 PID: 1 Comm: swapper/0 Not tainted
> 6.4.0-10173-ga901a3568fd2 #8
> > [? ? 3.093098] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
> rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
> > [? ? 3.093098] RIP: 0010:__mod_timer+0x1c3/0x370
> > [? ? 3.093098] Code: 00 00 41 bd ff ff ff ff 31 d2 4c 89 f6 4c 89 ff e8
> f2 ef ff ff 41 89 c4 85 c0 75 09 83 e3 01 0f 85 54 ff ff ff 41 8b 4f 20 66
> <90> f7 c1 00 00 10 00 0f 84 23 01 00 00 48 c7 c3 40 cc 01 00 65 48
> > [? ? 3.093098] RSP: 0018:ffffaf1600013e00 EFLAGS: 00000046
> > [? ? 3.093098] RAX: 0000000000000000 RBX: 0000000000000000 RCX:
> 0000000000280003
> > [? ? 3.093098] RDX: 0000000000000000 RSI: ffff9aa90fd9dec0 RDI:
> ffffffff8441e4b8
> > [? ? 3.093098] RBP: 00000000fffc200d R08: ffffffff8441e4a0 R09:
> ffffffff8441e4b8
> > [? ? 3.093098] R10: 0000000000000001 R11: 000000000002e990 R12:
> 0000000000000000
> > [? ? 3.093098] R13: 00000000ffffffff R14: ffff9aa90fd9dec0 R15:
> ffffffff8441e4b8
> > [? ? 3.093098] FS:? 0000000000000000(0000) GS:ffff9aa90fd80000(0000)
> knlGS:0000000000000000
> > [? ? 3.093098] CS:? 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [? ? 3.093098] CR2: 0000000000000000 CR3: 000000004e02e000 CR4:
> 0000000000750ee0
> > [? ? 3.093098] PKRU: 55555554
> > [? ? 3.093098] Call Trace:
> > [? ? 3.093098]? <TASK>
> > [? ? 3.093098]? ? die+0x31/0x80
> > [? ? 3.093098]? ? exc_int3+0x10e/0x120
> > [? ? 3.093098]? ? asm_exc_int3+0x39/0x40
> > [? ? 3.093098]? ? __mod_timer+0x1c3/0x370
> > [? ? 3.093098]? ? __mod_timer+0x1c3/0x370
> > [? ? 3.093098]? queue_delayed_work_on+0x23/0x30
> > [? ? 3.093098]? neigh_table_init+0x1bb/0x2e0
> > [? ? 3.093098]? arp_init+0x12/0x50
> > [? ? 3.093098]? inet_init+0x15b/0x2f0
> > [? ? 3.093098]? ? __pfx_inet_init+0x10/0x10
> > [? ? 3.093098]? do_one_initcall+0x58/0x230
> > [? ? 3.093098]? kernel_init_freeable+0x199/0x2d0
> > [? ? 3.093098]? ? __pfx_kernel_init+0x10/0x10
> > [? ? 3.093098]? kernel_init+0x15/0x1b0
> > [? ? 3.093098]? ret_from_fork+0x2c/0x50
> > [? ? 3.093098]? </TASK>
> > [? ? 3.093098] Modules linked in:
> > [? ? 3.093098] ---[ end trace 0000000000000000 ]---
> > [? ? 3.093098] RIP: 0010:__mod_timer+0x1c3/0x370
> > [? ? 3.093098] Code: 00 00 41 bd ff ff ff ff 31 d2 4c 89 f6 4c 89 ff e8
> f2 ef ff ff 41 89 c4 85 c0 75 09 83 e3 01 0f 85 54 ff ff ff 41 8b 4f 20 66
> <90> f7 c1 00 00 10 00 0f 84 23 01 00 00 48 c7 c3 40 cc 01 00 65 48
> > [? ? 3.093098] RSP: 0018:ffffaf1600013e00 EFLAGS: 00000046
> > [? ? 3.093098] RAX: 0000000000000000 RBX: 0000000000000000 RCX:
> 0000000000280003
> > [? ? 3.093098] RDX: 0000000000000000 RSI: ffff9aa90fd9dec0 RDI:
> ffffffff8441e4b8
> > [? ? 3.093098] RBP: 00000000fffc200d R08: ffffffff8441e4a0 R09:
> ffffffff8441e4b8
> > [? ? 3.093098] R10: 0000000000000001 R11: 000000000002e990 R12:
> 0000000000000000
> > [? ? 3.093098] R13: 00000000ffffffff R14: ffff9aa90fd9dec0 R15:
> ffffffff8441e4b8
> > [? ? 3.093098] FS:? 0000000000000000(0000) GS:ffff9aa90fd80000(0000)
> knlGS:0000000000000000
> > [? ? 3.093098] CS:? 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [? ? 3.093098] CR2: 0000000000000000 CR3: 000000004e02e000 CR4:
> 0000000000750ee0
> > [? ? 3.093098] PKRU: 55555554
> > [? ? 3.093098] Kernel panic - not syncing: Fatal exception in interrupt
> >
> > There are many variations, but the common pattern seems to be
> > <something in the clock or timer code> -> int3 exception
> >
> > It only happens under qemu TCG (software emulation).
> >
> > It goes away if we recompile qemu without MTTCG support.
> >
> > It only happens with -smp enabled, we are using qemu -smp 4
> >
> > We are using qemu-system-x86_64 full system emulation on x86_64 host
> > (ie. forcing KVM off).
> >
> > It happens with the latest upstream kernel and qemu, compiled from
> > source.
>
> I got a bit further on this one.
>
> The bug happens in the code that updates the static branch used for at
> least these two keys:
>
> ? static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
> ? DEFINE_STATIC_KEY_FALSE(timers_migration_enabled);
>
> There are probably others (it seems a generic problem with how static
> branches are handled by TCG), but I only see the bug for those two.
>
> When the static branch is updated we end up calling
> arch/x86/kernel/alternative.c:text_poke_bp_batch().? It's best to read
> the description of that function to see where int3 is used:
>
> ? https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/
> arch/x86/kernel/alternative.c#n2086
>
> I modified the qemu TCG int3 helper so it dumps the code at %rip when
> the interrupt fires, and I can actually see the changes in the above
> function happen, first int3 being written, then the end of the nop,
> then the int3 being overwritten with the first byte of the nop.
>
> Unfortunately the int3 still fires after the code has been completely
> rewritten to its final (ie nop) value.
>
> This seems to indicate to me that neither the self-write to the kernel
> text segment, nor sync_core (implemented by a "iret to self" trick)
> invalidates the qemu TCG translation block containing the old int3
> helper call.? Thus we (qemu) never "see" the new nop, we keep
> emulating int3, and that causes the kernel to crash.
>
> I added print statements inside tb_invalidate_phys_page() and this
> function seems never to be called at all.? It's my understanding that
> at least the kernel writing to its text segment ought to cause
> tb_invalidate_phys_page() to be called, but I'm not super-familiar
> with this qemu code.
>
> Richard Henderson - do you have any suggestions?
>
> Rich.
>
> --
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/
> ~rjones
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>
>
--
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